Add SshHostCollector for .180 (omv) and .171 (ripper)

Extends monitoring to the two bare-metal boxes Proxmox can't see.
Uses a dedicated ed25519 key with a forced authorized_keys command
(see docs/ssh-collector-key-setup.md) so a leaked key can only ever
run the fixed read-only stats script, never arbitrary commands.

CPU is approximated from 1-min load average / core count (a true
utilization % would need two /proc/stat samples).

Closes #8.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -7,8 +7,10 @@ services:
     environment:
       - HOSTS_CONFIG_PATH=/app/config/hosts.yaml
       - DB_PATH=/app/data/monitor.db
+      - SSH_PRIVATE_KEY_PATH=/app/ssh/monitor_ed25519
     volumes:
       - ./config/hosts.yaml:/app/config/hosts.yaml:ro
+      - ./ssh/monitor_ed25519:/app/ssh/monitor_ed25519:ro
       - monitor-data:/app/data
     networks:
       - web